On Solaris, uname() returns non-negative value

As per https://docs.oracle.com/cd/E18752_01/html/816-5167/uname-2.html#REFMAN2uname-2
Solaris version of uname() return a non-negative value on success. Modifying this code to check
for a return value of less than zero will work on Solaris as well as Linux and AIX.

src/logging.cc

@@ -238,7 +238,7 @@ static ssize_t pwrite(int fd, void* buf, size_t count, off_t offset) {
 static void GetHostName(string* hostname) {
 #if defined(HAVE_SYS_UTSNAME_H)
   struct utsname buf;
-  if (0 != uname(&buf)) {
+  if (uname(&buf) < 0) {
     // ensure null termination on failure
     *buf.nodename = '\0';
   }